1. Field of the Invention
The present invention relates to the field of forming conductor patterns, and more particularly, to forming conductors on substrates in patterns which are determined by laser illumination.
2. Prior Art
It is known from U.S. Pat. No. 4,340,617 to form a conductor pattern on a substrate via laser induced photodecomposition of a gaseous source material to deposit metal on the substrate in a pattern which corresponds to the pattern of the laser illumination. It is also known from U.S. Pat. No. 4,574,095 to use the technique of U.S. Pat. No. 4,340,617 to deposit a palladium metal activator on a substrate in the pattern of the laser illumination and then to selectively electrolessly plate copper on the activated portion of the substrate by immersing the substrate in an electroless copper plating bath. Both of these patents are incorporated herein by reference.
U.S. Pat. Nos. 4,340,617 and 4,574,095 both employ a laser which is focused adjacent to (not on the surface of) the substrate and depend on absorption of the laser light by the gaseous source material rather than the substrate. Both of these patents discuss their deposition process in a manner which makes no distinction between uniform substrates and substrates whose characteristics vary with position. Since both of these processes depend on decomposition of a gaseous material, both require that the deposition process be carried out in an enclosed container having a carefully controlled atmosphere. In a production environment, this limits system throughput and has attendant disadvantages.
U.S. Pat. No. 4,574,095 discloses use of its process with a substrate having a polymer layer thereon and touts as an advantage the fact that the laser beam causes removal of the polymer layer in the irradiated area and palladium metal deposition takes place in the same location. Upon electrolessly plating copper on that substrate, the copper conductor formed on the palladium is imbedded in the polymer and level therewith whereby the conductor is protected from mechanical damage. Thus, U.S. Pat. No. 4,574,095 teaches its process is not usable for forming a conductor pattern on top of a polymer layer since the process of catalyst deposition removes the polymer layer from the substrate. However, the removal of the polymer layer in the irradiated area is a disabling disadvantage where it is desired to form the conductor pattern on top of the polymer layer.
An article entitled "Laser Processing for Interconnect Technology" by H. S. Cole, Y. S. Liu, R. Guida and J. Rose which appears at SPIE Vol. 877, Micro-Optoelectronic Materials (1988), pages 92-96, of which the first two coauthors are the present inventors, describes a metal deposition process in which a solid source compound such as palladium acetylacetonate (PdAcAc) or palladium acetate (PdAc) is disposed on a substrate surface and laser decomposed to provide a palladium catalyst for electroless deposition of copper.
We have found that substantial problems which are not addressed in the above two patents and for which no solutions are disclosed in those patents are present when laser induced decomposition is applied to a substrate which has substantial non-uniformities in optical and thermal properties. An example of particularly severe changes in properties is a structure in which a polymer layer is disposed over a substrate and has via holes therein disposed in alignment with a metal layer on the substrate and it is desired to form a conductor pattern which is disposed on top of the polymer layer and which extends down into via holes and onto the metal which is exposed at the bottom of the via holes. The polymer layer and the metal layer at the bottom of a via hole have substantially different optical reflectivities, optical absorptivities and thermal conductivities. As a result of these substantial differences in characteristics, a laser beam scan rate which is effective on top of the polymer layer is ineffective at the bottom of the via hole. We have also found that an increase in power density or a reduction in the scan rate on the metal layer at the bottom of the via hole which is sufficient to cause catalyst source material decomposition on that metal is destructive to the directly adjacent portions of the polymer layer.
A process which will reliably and uniformly deposit a metallization pattern both on the top surface of a polymer film and on the walls of via holes in that film and on metallization exposed at the bottom of those via holes is needed.